Expanded materials or foams containing amorphous or atactic polystyrene have been widely used in a variety of applications because of their low density, low thermal conductivity, low cost and good energy absorption properties. Particular applications include thermal insulators, disposable containers, and protective packaging.
Most of the important properties of polystyrene, including energy absorption, thermal resistivity, strength to weight ratio, and cost per unit volume, are strongly dependent on density and, therefore, can be easily adjusted by controlling the amount of expansion. However, expanded polymeric materials other than polystyrene foam are often selected where certain nonstrength properties are important, particularly high temperature resistance.
It is known that syndiotactic polystyrene exhibits certain improved heat resistant properties as compared with atactic polystyrene. Specifically, syndiotactic polystyrene has a higher melting point than atactic polystyrene. However, the heat distortion temperature, an important thermal property for expanded polymeric materials, is only about 80.degree. C. for compositions comprised of substantially pure syndiotactic polystyrene. The patent literature suggests that various rubber-like polymers and/or thermoplastic resins can be blended with syndiotactic polystyrene to control viscosity and the rate of crystallization. In particular, it has been disclosed that atactic polystyrene, isotactic polystyrene, polyphenylene ethers, or mixtures thereof are compatible with syndiotactic polystyrene, and that by selecting the amount, type and molecular weight of these polymers, the expansion molding methods used for conventional crystalline resins, such as polyethylene and polypropylene, and for conventional non-crystalline resins, such as atactic polystyrene and polyvinyl chloride, can be applied. The patent literature also discloses that syndiotactic polystyrene can be blended with polyphenylene ethers to provide compositions endowed with enhanced mechanical properties as compared to those containing amorphous polystyrene or isotactic polystyrene. However, the literature does not suggest that polyphenylene ethers or any other materials can be added to syndiotactic polystyrene to improve the mechanical properties thereof at higher temperatures.